The present invention concerns magnetic resonance (MR) imaging, and in particular concerns simultaneous multi-slice (SMS) MR imaging.
Description of the Prior Art
MR imaging is a widely used imaging modality for medical diagnosis as well as for material inspection.
In a magnetic resonance apparatus, the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field, by the operation of a basic field magnet of an MR scanner, in which the examination object is situated. The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals. The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object, and are therefore often called excitation pulses. The excitation of the nuclear spins at an appropriate frequency gives the excited spins a magnetization that causes the nuclear spins to deviate, by an amount called the flip angle, from the alignment of the nuclear spins that was produced by the basic magnetic field. As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals), which are received by suitable RF reception antennas in the MR scanner, which may be the same or different from the RF radiator used to emit the excitation pulse.
The emitted MR signals have a signal intensity that is dependent on the exponential decay over time of the magnetization of the nuclear spins. The acquired signals are digitized so as to form raw data, which are entered into a memory that is organized as k-space, as k-space data. Many techniques are known for reconstructing an image of the examination object from the k-space data.
By appropriately selecting different characteristics of the MR data acquisition sequence that is used, the acquired signals can be differently weighted so that different sources of the detected MR signals (i.e., different tissues in the case of medical MR imaging) appear with different contrasts in the reconstructed image. In the case of medical MR imaging, a weighting is selected that causes the tissue that is important for making the intended medical diagnosis to have the best contrast (brightness) in the reconstructed image. One such type of weighting is known as T1-weighting, because it depends on the so-called T1 relaxation time of the nuclear spins.
Many different techniques are known for acquiring the raw MR data. One such technique is known as simultaneous multi-slice (SMS) acquisition, which is a technique for accelerating the acquisition of the data from a given volume of the examination object, wherein nuclear spins in multiple slices are excited simultaneously, and the resulting MR signals are simultaneously acquired from each slice. This results in a dataset in k-space that is composed of data from the multiple slices collapsed on top of each other. Techniques are known for separating or uncollapsing the data for these respective slices during image reconstruction, such as the slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) technique, which is schematically illustrated in FIG. 1. In the example shown in FIG. 1, multiple slices S1, S2 and S3 are excited simultaneously, resulting in each slice generating an echo train of magnetic resonance signals, which are acquired according to the known blipped CAIPIRINHA (Controlled Aliasing in Parallel Imaging Results in Higher Acceleration) technique. Details of such techniques are described, for example, in Setsompop et al., “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging With Reduced g-Factor Penalty,” Magnetic Resonance in Medicine, Vol. 67, pp. 1210-1224 (2012) and Setsompop et al., “Improving Diffusion MRI Using Simultaneous Multi-Slice Echo Planar Imaging,” Neurolmage, Vol. 63, pp. 569-580 (2012) and Cauley et al., “Interslice Leakage Artifact Reduction Technique for Simultaneous Multislice Acquisitions,” Magnetic Resonance in Medicine, Vol. 72, pp. 93-102 (2014).
Excitation of the nuclear spins in the simultaneously acquired slices is implemented with a multi-band (MB) RF pulse. An MB RF pulse is generated by the superimposition of a number of individual single band (SB) RF pulses, of the type that are typically used to excite nuclear spins in a single selected slice in conventional magnetic resonance imaging.
The turbo spin echo (TSE) sequence is the “clinical workhorse” sequence for MR imaging, by virtue of being the most utilized sequence for all types of body region imaging. A TSE sequence has several echo trains, and in each echo train, multiple phase encoding lines of the entirety of k-space are scanned (filled with data) after one excitation pulse. This is achieved by refocusing the spins after each readout line, utilizing refocusing RF pulses. Compared to a conventional spin echo (SE) sequence, the acquisition time in a TSE sequence is reduced by the number of refocused echoes in one echo train. This reduction is known as the turbo factor.
FIG. 9 shows a sequence diagram for one echo train of a conventional TSE sequence, specifically a T1-FLAIR TSE protocol. In this conventional protocol, data are acquired from 15 slices in two concats of 7 and 8 slices respectively, with a turbo factor 7. Dead time 1 and dead time 2 are shown in FIG. 9, following the IR (inversion recovery) pulses for slice 1 and slice 2, and following the echo train.
It is known to combine SMS and TSE, in order to acquire data from two or more slices simultaneously. This reduces the minimum repetition time (TR) which is given by the length of all echo trains for all slices that are executed back-to-back. The reduction occurs because fewer slices must be acquired with such a combination. The total number of reduced slices is known as the slice acceleration factor. For many examinations, however, the minimum TR is not limited by the total time of all echo trains, but instead is limited by the desired image contrast.
Therefore, some TSE protocols are designed so as to intentionally have dead time intervals therein, during which no imaging is performed. For typical examinations, several of these protocols must be used, which increases the total amount of unused (dead) time. One example of a common workflow that is affected by this problem is spine imaging, wherein typically a sagittal T1 dark fluid, as well as a T2 contrast, must be acquired. Realistic settings of the current protocols are 15 slices with TR=2,000 ms, TE=9 ms, turbo factor 6, matrix size 320 for the T1 dark fluid protocol with a dead time of 800 ms per echo train, which results in a total acquisition time of 4 minutes and 38 seconds. Realistic settings for the T2 contrast acquisition are TR=3,500 ms, TE=95 ms, turbo factor 17, matrix size 384 with a dead time of 2,000 ms per echo train, which results in a total acquisition time of 3 minutes and 46 seconds. Because both protocols must be executed consecutively, approximately 50% of the total acquisition time is not used for imaging (i.e., raw data acquisition).
Applying SMS to these known techniques will not result in reduced measurement time, because in that context the use of SMS imaging would only increase the dead time duration, but would not reduce the total acquisition time.